This invention relates to a method for installing cables in tubes, in which the cable is installed by the combination of a pushing force and the drag force of a fluid under pressure which is passed through the tube.
Such a method is known from EP-A-0 292 037.
The known technique has as an advantage that the forces propelling the cable are at least partly distributed over the length of the cable, so that, in practice, the cable can be installed over long distances, also in the case of winding paths. EP-A-0 292 037 discloses using a gas stream as fluid, while GB-B-2,122,367 describes the use of a liquid stream.
In the conventional installation of cables by pulling, the force is fully concentrated at the forward end of the cable and the installation in winding paths proceeds with considerably more difficulty.
When cables are to be installed in tubes of a diameter of a few millimeters in a winding path, installation using a gas under pressure is possible only to a certain tube length. One of the causes is that the cables adhere to the inner wall of the tube as a result of electrostatic charge arising, during propulsion, from the friction between the outer wall of the cable and the inner wall of the tube. This problem is partly solved by the use of a liquid instead of a gas, in that the cables float on the liquid. In the case of tubes of small diameters, however, the flow velocity of the liquid, and hence the speed of propulsion of the cable, becomes very low when longer installation lengths are involved. Installation times of many hours are no exception then. This obviously impedes efficient installation of the cables. In addition, the stiffness of the cable, in particular the stiffness in the forward end of the cable when passing through bends, plays an important role. This is true in particular if the cable just floats on the liquid.
The present invention contemplates a solution to the problems outlined and, to that end, provides a method of the above-mentioned kind, in which, during the installation, as fluid, alternately a first medium having first hydrodynamic properties is passed into the tube and at certain moments a second medium having second hydrodynamic properties.
According to the invention, a difference in hydrodynamic or flow properties is defined as follows: if a section of a flowing medium is considered separately and, in a specific tube having a pressure p1 at the inlet end and a pressure p2 at the other end, results in a different flow velocity than does a section of a second flowing medium in the same tube under the same conditions, different hydrodynamic properties are involved. This may involve different gases or different liquids having different viscosities and/or densities, or gases and liquids may be involved. What may also be involved is a gas or liquid in which sections of different temperatures, temperature pulses, are introduced and/or to which, in part, a for instance solid substance is added.
According to the invention, in a preferred embodiment, the first medium is a gas stream and the second medium is a liquid.
According to a second aspect of the invention, the liquid has a good electrical conductivity.
The measures according to the invention have the favorable influence elucidated hereinafter on the above-described problems of slow installation speed, cable stiffness and adhesion of the cable to the tube wall.
Since gases generally have a lower viscosity than liquids, the combination of a gas with a liquid will flow faster than the liquid alone. Surprisingly, by selecting as liquid a liquid of a sufficient electrical conductivity, the problem of cable adhesion to the tube wall is also solved, since any electrical charge produced will be immediately removed owing to the conductivity of the liquid. In this exemplary embodiment of the invention, in the tube in which a cable is being installed, alternately, gas will be present over a part of the tube length and a liquid over a next part. In such a situation, the greater pressure gradient is present in the part with the liquid, in that the viscosity of the liquid is higher than that of the gas. As a consequence, areas of different pressure gradients move through the tube, so that at certain moments the cable is subjected to a greater force, which, for instance, is just sufficient for the forward end of the cable to pass through a bend. Under certain circumstances, it is even possible for the cable to move through the tube making a xe2x80x9cserpentinexe2x80x9d, movement, which further increases the attainable installation length.
The present invention further provides an apparatus for installing a cable in a tube, comprising a feeder unit with a feed-in opening for feeding in a cable end, at least two pressure rollers for engaging the cable to transport it from the feed-in end to the tube, a feed-out opening for the cable, to which feed-out opening the tube can be connected, and a supply opening for supplying a first medium having first hydrodynamic properties and a second medium having second hydrodynamic properties, and means for alternately supplying the first and the second medium via the feeder unit to the tube.